The Iris-Diaphragm unit including the servo-motor to open/close the Iris

The variable focal length of a zoom lens is normally expressed as a ratio. A lens with a focal length of 24mm at the wide end and 105mm at zoomed (narrow) end of the range is expressed as a 24:105mm. Lenses with an easily calculated ratio are expressed as a proper ratio. So a 100mm to 200mm focal length range will be expressed as 1:2 or 2x zoom (eg. a 2 times zoom).

Zoom lenses which may be calculated as more than 4x are sometimes called superzoom lenses.

Some digital cameras, mostly point-and-shoot or bridge cameras, can digitally emulate a zoom effect. This ‘digital zoom’ is not a true zoom. Instead the camera will crop an image so that the objects that are zoomed in upon are of a lower digital pixel density. This creates a picture of lower resolution resulting in a lower quality image than would be produced with a proper optical lens.

Zoom lenses provide a whole range of effects. Within the short focal lengths are the wide angle zooms at the wide end of the focal range (24mm and less). The latter give a view wider than the normal view.

Some zooms provide coverage from wide angle to long focus. These have focal lengths ranging from around 35mm to 85mm and are sometimes called ‘normal zooms’.

It is the normal zooms group of lenses in particular that have provided the flexibility on which manufacturers have beased a number of popular mid-range zooms used by amateurs and hobbyists. They often constitute sufficient zoom coverage that many users find no need to buy other lenses. Once the province of the prime lens this group of lenses have taken most of the market in this focal length range. These popular zooms were originally based on a focal length range of 24 to 120mm. However, lens manufacturing has improved lens production so much that now it is not unusual to have 24 – 200mm (or longer) lenses without significant distortion. This has pushed the popularity of the zoom lens even further into the public domain.

In the longer focal length range of lenses are the ‘long focus’ zooms. These are where the focal length is greater than the normal lens and creates a magnification. These types of lenses often have specialised lens formula so that one or more lenses act to create a telephoto effect.

By Damon Guy (author and Photokonnexion editor)

Damon Guy (Netkonnexion)

Damon is a writer-photog and editor of this site. He has run some major websites, a computing department and a digital image library. He started out as a trained teacher and now runs training for digital photographers.See also: Editors ‘Bio’. See also: Profile on Google+.